540 



SCIENCE 



[N. S. Vol. LIV. No. 140-5. 



ations of the constitution of the vegetation 

 when the other factors remain the same, 

 whereas habitats with about the same concen- 

 tration of hydrogen ions and equal with re- 

 gard to light and moisture carry about the 

 same vegetation. When the material collected 

 was statistically investigated, it was further 

 proved that many species are only found on 

 soil where the concentration of hydrogen ions 

 is within a certain range of concentration of 

 hydrogen ions characteristic for each single 

 species. "Within this is found another range 

 with narrower limits, within which the species 

 has its largest average frequency. It was 

 further proved that it was iwssible to judge- 

 of the concentration of hydrogen ions in the 

 soil from the constitution of the plant forma- 

 tions, when they did not consist of too few 

 species; this holds good, for instance, for 

 meadows. 



The number of species found and the den- 

 sity of species (the number of species found 

 on 0.1 sq. m.) were on the whole largest on 

 soil near the neutral point; number of spe- 

 cies and density of species become generally 

 less as the concentration of hydrogen ions in 

 the soil increases. 



By a series of water-culture experiments 

 it was proved that the species which are found 

 only on very acid soil (acid soil plants) show 

 the strongest growth in culture media with 

 pH values near 4, whereas species which 

 naturally grow only in soils that are neutral 

 or but slightly acid or basic (alkaline soil 

 plants) have the strongest growth in culture 

 media, the pH values of which are between 

 6 and 7. In the slightly acid culture media 

 in which the basic soil plants have their 

 strongest growth the acid soil plants thrive 

 badly and become chlorotic. 



According to the theory of Hartwell and 

 Pember^ basic soil plants can not thrive in 

 very acid soils, not because these plants can 

 not stand so high a concentration of hydrogen 

 ions as the acid soil plants, but because the 



1 Hartwell, B. L., and Pember, F. E., 1918, ' ' The 

 presence of aluminum as a reason for the differ- 

 ence in the effect of so-called acid soil on barley 

 and rye, ' ' Soil Science, 6, 259. 



very acid soils contain small quantities of 

 dissolved aluminum compounds, which are 

 said to be poisonous for the basic soil plants 

 and not for the acid soil plants. This theory 

 has been proved not to be generally valid, as 

 experiments have shown that aluminum ions 

 are not poisonous for all basic soil plants, 

 generally speaking. 



According to Bear^ and others acid soil 

 plants can make use of the nitrogen in am- 

 monia, whereas basic soil* plants require 

 nitrate nitrogen, which makes it impossible 

 for them to thrive in very acid soil in which 

 nitrification is weak or wanting. Experi- 

 ments showed that nitrogen from ammonia 

 and from nitrate nitrogen are of the same 

 value for acid soil plants and for basic soil 

 plants, when the plants were cultivated at 

 constant pH. If on the other hand the pH 

 is not kept constant, the plants make the 

 solution more acid, when the source of nitro- 

 gen is a salt of ammonia (including thereby 

 ammonia). In this case the basic soil plants 

 soon die, because the solution becomes too 

 acid. The acid soil plants on the other hand 

 last longer as they are more tolerant of acid. 

 If the source of nitrogen is a nitrate (nitrate 

 of ammonia excepted), the plants make the 

 solution more alkaline and the plants die, 

 after having first become chlorotic. The 

 chlorosis takes place for acid soil plants when 

 the pH value of the culture medium has 

 the culture medium has reached a pH value 

 reached 6.0, but for basic soil plants not till 

 of about 7.0. 



The investigations prove that the quantity 

 of nutritive substances does not largely in- 

 fluence the distribution of plants. This is 

 opposed to the results of some investigators, 

 who consider that the acid soils are poor and 

 the neutral and basic soils rich in such sub- 

 stances. It has been proved that basic soils 

 exist which are very poor in nutritive sub- 

 stances, and their vegetation does not re- 

 semble that of very acid soils, which are poor 

 in nutritive substances. 



2 Bear, F. E., 1917, ' ' A correlation between bac- 

 terial activity and lime requirement of soils," Soil 

 Science, 4, 435. 



